documentclass[]{article}
usepackage{tikz}
usetikzlibrary{graphs}
usetikzlibrary{positioning}
begin{document}
    begin{tikzpicture}[SCbox/.style={rectangle,draw=black}]

    graph[grow right sep=10mm]
    {
        A[SCbox] ->
        B[SCbox];
    };

    node (C) [SCbox, below=of B] {C};

    graph[grow right sep=10mm]
    {
        (C) ->
        D[SCbox];
    };

    end{tikzpicture}
end{document}